Squats and Deadlifts up to your body weight should be fairly easy for any otherwise healthy male.

Best thing I've learned from bodybuilding and yoga - listen to your body. If you feel any discomfort beyond normal muscle contractions - stop, rest, and come back tomorrow. Do slower, more controlled movements that tax your muscles more than your joints.

Calisthenics is generally very safe as well (compared to powerlifting), and you can build up an impressive physique doing it.